1 Introduction

Metalaxyl is a fundamental benzenoid fungicide, and can be immensely applied to the plant diseases caused by Oomycete fungi [1]. Its IUPAC name is methyl N-(2, 6-dimethyl-phenyl)-N-(methoxyacetyl)-alaninate and molecular weight is 279.33 g/mol. The molecular structure of metalaxyl is shown in Fig. 1. This fungicide is very active across soil-borne and foliaceous diseases. It is continuously used in agricultural purpose because of its great tolerance for light, temperature and pH of soil [2]. Metalaxyl is frequently sprayed on plant after dilution [3]. It has long half-life in soil and it is greatly soluble in water (solubility: 8400 mg/L) which can issue a great threat of environment [4, 5]. The excessive application of metalaxyl can stimulate the contamination of soil and ground water, owing to high mobility and a low soil adsorption of metalaxyl [6, 7]. In addition, metalaxyl is characterized as a stable compound in environment for the hydrolysis at normal pH values [8, 9]. These behaviors present the persistence and toxicity of metalaxyl in the environment. In addition, the minimum detection limit (MDL) of metalaxyl is tentatively 0.01 μg/L based on the report, provided by effluent characterization program [10]. Admittedly, up to 0.49 mg/L of metalaxyl, which exceeds the 0.1 mg/L EU limit, has been recorded in groundwater [11]. Therefore, the degradation of metalaxyl is essential to control its toxicity in the environment. As far, the degradation of metalaxyl has been reported by some traditional, physiochemical methods such as, micro-organisms [12], Mucorales strains [13], sorption–desorption process [14], biotic process [15] and filamentous fungi [16], etc. These traditional physical or chemical methods have some major drawbacks such as costly, ineffective, time consuming and an incomplete degradation [17]. For example, Sukul et al. [14] has reported that a significant amount of metalaxyl kept tightly bound to the adsorbents after desorption cycle. Thus, desorption of 22–56% of the total amount of the retained metalaxyl was even determined. Therefore, many researchers have started to find new method for the elimination of pesticides and other pollutants from water and soil. Among the researches, the photocatalytic degradation with solar irradiation or artificial light irradiation has been considered as an effective techniques [18,19,20]. Photocatalytic degradation technology not only manifests a high degradation efficiency and low energy consumption with low cost, but also converts hazardous pollutant into the inorganic compound such as CO2, water and mineral acids [21]. Conventionally, the photocatalytic method involves the migration of valence electrons to conduction band, when photocatalyst are irradiated under light having photons with more energy than bandgap of the photocatalyst [22]. So far, some significant photocatalysts, namely TiO2, ZnO, CuO, WO3, CdS, Fe2O3, ZnFe2O4, etc., were widely studied due to favorable band gap energy to be excited under UV irradiation, low cost and suitable chemical properties [23,24,25,26,27]. TiO2 and ZnO photocatalysts are highly applicable owing to their nontoxicity, wide bandgap (Eg = ~3.20 eV) and high photosensitivity [28]. However, some limitations, such as the poor stability of photocatalysts except for TiO2 in strong acidic medium, are found alongside all advantages [20, 29]. Furthermore, photocatalysts can be recycled and reused repeatedly after completing photocatalytic degradation process [30, 31]. A very few works have been reported on the photocatalytic degradation of metalaxyl under UV or solar irradiation using semiconductor based metal oxide [32, 33]. Recently, visible–light–driven photocatalytic degradation of metalaxyl with grapheme oxide/Fe3O4/ZnO ternary nanohybrid was reported [10]. The solar photocatalytic treatment of wastewater is cheap, simple and environmental-friendly, compared with that using artificial lamp devices such as Hg–Xe lamp. Therefore, the solar photocatalytic degradation of metalaxyl with irradiated semiconductor photocatalyst can be considered as an effective method for degradation and mineralization of metalaxyl.

Fig. 1
figure 1

Molecular structure of metalaxyl

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The present study demonstrates that the photocatalytic degradation of metalaxyl aqueous solution with TiO2 photocatalyst under solar irradiation. Gas chromatography-mass spectroscopy (GC/MS) has been used to identify the intermediate products of metalaxyl. A probable reaction pathway has been drawn, depended on the generation of intermediate products. The different parameters such as catalyst dosage, pH, temperature, irradiation intensity and time have been optimized for the photocatalytic degradation of metalaxyl.

2 Experimental section

2.1 Chemicals and materials

Metalaxyl (C15H21NO4) and hexane were purchased from FUJIFILM Wako Pure Chemical Industries Ltd., Japan. TiO2 (P–25) was obtained from Nippon Aerosil Co. Ltd., Japan. Potassium hydrogen phthalate (C6H4(COOH)(COOK)), methanol (CH3OH), dichloromethane (CH2Cl2), sodium carbonate (Na2CO3), sodium bicarbonate (NaHCO3) and ammonium chloride (NH4Cl) were purchashed from Nacalai Tesque Co. Ltd., Japan. NaOH and H2SO4 were used to adjust solution pH.

2.2 Photodegradation of metalaxyl

The required amount of metalaxyl powder was dissolved with ultrapure water, obtained from an ultrapure water device (GSH-2000, Advantech Tokyo Co., Ltd.), to prepare 1000 ppm stock solution of metalaxyl. For a typical photocatalytic run, 30 mL of 50 ppm metalaxyl solution was taken into a 50 mL Pyrex reaction vessel, and 30 mg TiO2 powder was added into metalaxyl solution. The pH of the solution was adjusted with 1 M NaOH and 1 M H2SO4. The temperature was kept constant with a water bath. The metalaxyl solution containing TiO2 photocatalyst was allowed to equilibrate for 30 min in dark. Then, it was irradiated under sunlight. The suspension was continuously stirred by magnetic stirrer for the dispersion of TiO2 during the treatment. The vessel wall filtered the short ultraviolet radiation (λ < 300 nm). The irradiance was measured by a UV intensity meter with sensor of 230–400 nm wavelengths (UVR-400, Iuchi Co., Osaka, Japan). The experimental set up is presented in supplementary material (Fig. S1). The 0.45 μm Advantec membrane filter was used to separate TiO2 after irradiation. The high-performance liquid chromatograph equipped with a UV optical detector and ODS-3 (5 μm) column was used to evaluate the amount of metalaxyl in the aqueous solution. The elution was observed at 220 nm. The mixture of acetonitrile and water (50/50, v/v) was considered as the eluent with flow rate of 1 mL/min. The injected sample volume was 20 μL. Then, the amounts of converted NH4+ and NO3− ions were measured by an anion ion chromatography after separating TiO2 from reaction solution. The detailed analytical conditions are presented in Table S1 and S2. The degradation efficiency of metalaxyl was calculated by the following equation

$${\text{Degradation}}\,{\text{efficiency}} = \left( {\frac{{C - C_{0} }}{{C_{0} }} \times 100\% } \right)$$

where, C and C0 are the concentration of metalaxyl before and after degradation.

For total organic carbon (TOC) measurement, the mixed gas (O2 + N2) was passed for 30 min to remove inorganic carbon from the sample solution, and finally, the aqueous solution was subjected to determine TOC (Shimadzu TOC analyzer, TOC-VE). The calibration curve TOC was prepared by preparing a solution of known concentration using potassium hydrogen phthalate.

Molecular orbital (MO) calculations were carried out at the single determinant (Hartree–Fock) level for optimization of the minimum energy obtained at the AM1 level. All semi-empirical calculations were performed in MOPAC Version 6.01 with a CAChe package (Fujitsu Co. Ltd.). The partial charge and the frontier electron density at each atom on the molecule were determined. The intermediate products were evaluated by the solid-phase extraction gas chromatography and mass spectroscopy (GC–MS). A Shimadzu gas chromatography mass spectroscopy (GCMS-QP5000, Shimadzu) equipped with a CP-Sil 8 CB capillary column was used as the chromatographic conditions of Table 1.

Table 1 Experimental conditions for GC–MS
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3 Result and discussion

3.1 UV–Vis spectral changes

The change of the UV absorption spectrum during the photocatalytic decomposition of metalaxyl by TiO2 under UV irradiation has been investigated in the range of 220–320 nm by using a UV–Vis spectrophotometer. The detailed experimental conditions are presented in supplementary material (Table S3). As shown in Fig. 2, a little change in the absorption spectrum was observed after the 30 min solar irradiation without TiO2 (curve b), compared to that before irradiation (curve a). Therefore, the decomposition of metalaxyl did not occur only under solar irradiation. However, the absorption spectra has been changed when TiO2 was added under solar irradiation (curve c, d, e) relative to that before irradiation (curve a). Although the change in spectrum range of more than 280 nm indicated the intermediate formation of metalaxyl, the typical absorption peak (267 nm and 274 nm) for the metalaxyl decreased on using TiO2 under solar light. Hence, this result confirms the decomposition of metalaxyl by TiO2 under solar irradiation.

Fig. 2
figure 2

UV absorption spectra of metalaxyl before and after solar irradiation. Metalaxyl: 50 ppm; TiO2: 20 mg; light intensity: 2.2 mW/cm2; temperature: 20 °C and pH 6

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3.2 Effect of photocatalyst dosage

The effect of TiO2 amount on the photocatalytic degradation of metalaxyl has been examined, in the range of 0–50 mg TiO2. The results are shown in Fig. 3. The degradation efficiency was increased sharply with increasing up to 20 mg of TiO2 and, then an almost steady result was observed in the further addition of TiO2. Beyond a certain limit of photocatalyst amount, i.e. 20 mg of TiO2, the reaction solution could become turbid and thusly hinder the light penetration to proceed the reaction and therefore, percentage of degradation did not increase [34, 35]. Another possible reason could be due to the constant in the portion of the irradiated surface of the photocatalyst, the obstruction of light in the dense slurry and a loss in surface area by agglomeration (particle–particle interactions) at high solid concentration [36, 37]. Therefore, 20 mg of TiO2 was selected as the optimal amount for the subsequent experiments.

Fig. 3
figure 3

Effect of TiO2 dosage on the photocatalytic degradation of metalaxyl under solar irradiation. Metalaxyl: 50 ppm; irradiation time: 10; pH 6; light intensity: 2.2 mW/cm2 and temperature: 20 °C

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3.3 Effect of pH

The effect of pH is addressed as an important parameter during photocatalytic degradation process. The surface charge of the photocatalyst may vary with the pH value of the solution. Therefore, the effect of pH on the photodegradation of metalaxyl by TiO2 under 10 min solar irradiation was examined in pH range from 2 to 10. It is noteworthy to mention that the initial pH of the reaction solution was 6 without adjustment. As shown in Fig. 4, the highest 76% degradation efficiency was recorded at pH 8. The degradation efficiency was almost the same for pH 4–7. By considering the treatment cost and environmental safety, it was desirable to decompose metalaxyl near neutral pH.

Fig. 4
figure 4

Effect of pH on the photocatalytic degradation of metalaxyl under solar irradiation. Metalaxyl: 50 ppm; TiO2: 20 mg; irradiation time: 10 min; light intensity: 2.2 mW/cm2 and temperature: 20 °C

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The adsorption of metalaxyl onto TiO2 surface has been measured from the stimulation of molecular partial charges (Table 2). The most negative partial charged atoms in metalaxyl are the two oxygen atoms (13O and 18O) along 9 N nitrogen atom, while the most positive partial charged atom is carbon atom (11C). The point zero charge of TiO2 is approximately 6.5. Therefore, TiO2 surface is predominantly positively charged below this pH and negatively charged above pH 6.5 [38]. Hence, because of the electrostatic attraction, the positively charged Carbon 11C atom is readily adsorbed on the TiO2 surface in alkaline media (pH > 6). Contrarily, the negatively charged 13O, 18O and 9 N atoms can be adsorbed on TiO2 in acidic condition (pH < 6). Moreover, high initial pH states that more hydroxyl ions are present in solution. As a result, hydroxyl ions would be oxidized to hydroxyl radicals (·OH) by the holes forming on TiO2 surface [39]. The photocatalytic degradation of metalaxyl can be enhanced in alkaline medium, since ·OH is the dominant oxidizing species in this photocatalytic degradation process. Therefore, pH 6 was selected for the optimal conditions to avoid the unwanted chemical treatment.

Table 2 Calculation of frontier electron density and point charge of metalaxyl
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3.4 Effect of temperature

The effect of reaction temperature on the photocatalytic decomposition of metalaxyl was observed in the range of 10–60 °C. Because metalaxyl was not completely decomposed, the irradiation time was set to 10 min in order to check the influence of the reaction temperature. As shown in Fig. 5, the decomposition efficiency of metalaxyl tended to increase gradually as the reaction temperature increased, and the phenomena is comparable to other research [40]. This result concluded that a higher temperature is better for the decomposition of metalaxyl. However, the energy cost requires for raising the temperature during practical application. Therefore, the reaction temperature was set to 20 °C, because the effect of temperature was not significant in this experiment.

Fig. 5
figure 5

Effect of temperature on the photocatalytic degradation of metalaxyl under solar irradiation. Metalaxyl: 50 ppm; TiO2: 20 mg; irradiation time: 10 min; light intensity: 2.2 mW/cm2 and pH 6

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3.5 Effect of light intensity

The intensity of solar irradiation is greatly dependent on the latitude, season and time of treatment place and is related with the weather. Considering these factors, the effect of light intensity on the photo-degradation of metalaxyl with TiO2 is essential. The photocatalytic degradation of metalaxyl was conducted under sunlight with different intensities of light in different time of sunny and cloudy days. As shown in Fig. 6, the decomposition efficiency of metalaxyl gradually increased as the light intensity increased. Generally, semiconductor based photocatalyst absorbs the light with an equal or more than band gap energy. The excitation of photocatalyst can be motivated by raising the incident light intensity [41]. Therefore, the degradation of metalaxyl is slow at low intensity of light, because of recombination of the hole-electron. On the other hand, the recombination can be prohibited by using higher light intensity [42]. Therefore, an accelerated photodegradation could be achieved with increasing the intensity of incident radiation.

Fig. 6
figure 6

Effect of light intensity on the photocatalytic degradation of metalaxyl under solar irradiation. Metalaxyl: 50 ppm; TiO2: 20 mg; irradiation time: 10 min; pH 6 and temperature: 20 °C

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3.6 Effect of light irradiation time

The effect of the light irradiation time on the photocatalytic degradation of metalaxyl with optimum experimental conditions was examined. From Fig. 7a, the degradation efficiency of metalaxyl sharply increased with increasing irradiation time in the presence of TiO2. The degradation efficiency reached 100% with 20 mg of TiO2 within 30 min at 20 °C. It was found from these results that the initial structure of metalaxyl was completely broken after 30 min of light irradiation. If the irradiation time increases, the interaction of metalaxyl molecule with the surface of TiO2 will also be increased. Therefore, the photodegradation efficiency was increased eventually as shown in other research [43].

Fig. 7
figure 7

a Effect of irradiation time on the photocatalytic degradation of metalaxyl under solar irradiation; b Plot of − Ln(C/C0) versus irradiation time. Metalaxyl: 50 ppm; TiO2: 20 mg; light intensity: 2.2 mW/cm2; pH 6 and temperature: 20 °C

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3.7 Kinetics

The kinetics of photocatalytic degradation process have been studied according to the Langmuir–Hinshelwood (L–H) model [44], and this model was motivated by Turchi and Ollis [45]. The photocatalytic degradation of metalaxyl with TiO2 is postulated to be consistent with the Langmuir–Hinshelwood (L–H) model and expressed as Eq. 1

$$r = - \frac{dC}{dt} = \frac{kKC}{1 + KC}$$
(1)

where, r is the degradation rate of the reactant, k is the rate constant, K is the adsorption co-efficient and C is the substrate concentration. When the initial concentration C0 is very small (Eq. 1), it can be simplified to (Eq. 2), and becomes a linear expression of time t with respect to Ln(C/C0), here, kapp is reaction rate constant.

$$- Ln\left( {\frac{C}{{C_{0} }}} \right) = kKt = k_{aap} t$$
(2)

So as to confirm the speculation, − Ln(C/C0) was plotted as a function of the treatment time (irradiation time) for the photocatalytic decomposition and photolysis of metalaxyl. Because the liner relations were obtained in Fig. 7b as expected, the reduction kinetics of photodegradation of metalaxyl could follow pseudo-first-order kinetics, the fact was consistent with the Langmuir–Hinshelwood model, resulting from the low coverage in the experimental concentration range (50 ppm). The pseudo-first-order reaction rate constants for photocatalytic decomposition of metalaxyl was 0.105 min−1, and the half-life was 6.60 min. The kinetic studies confirmed that photocatalytic decomposition of metalaxyl by TiO2 was effective because it proceeded by the photolysis reaction of metalaxyl.

3.8 Mineralization

As shown in Fig. 7, metalaxyl completely decomposes in presence of TiO2 after 30 min of solar irradiation. Many intermediates of metalaxyl were detected in this degradation process. Therefore, it is necessary to examine the mineralization of metalaxyl. The photocatalytic mineralization of nitrogen containing organic compounds predominantly depends on the chemical nature of the organic pollutant, that is, the oxidation state and the position of nitrogen atoms in molecular chain [46, 47]. It has been reported that a complete oxidation of nitrate ions does not occur during the mineralization of organic compounds containing N. In the mineralization, ammonium ion can also be generated [48, 49]. Probably, ammonium ion, nitrate ion and CO2 can be considered as the mineralization product of metalaxyl. A hypothesized equation is shown here for total mineralization reaction of metalaxyl solution with TiO2 nanoparticles, assuming that the ammonium and nitrate ions were generated from nitrogen atom of metalaxyl.

$$2C_{15} H_{21} NO_{4} + 37O_{2} \to NO_{3}^{ - } + NH_{4}^{ + } + 30CO_{2} + 19H_{2} O$$
(3)

The amount of produced ammonium ion and nitrate ion from mineralization of nitrogen atom of metalaxyl, was investigated. The productions of nitrate and nitrite ions could not be confirmed by ion chromatography. The amount of produced ammonium ion was increased with the light irradiation time, and the total nitrogen yield became constant at about 86% after 6 h (Fig. 8a). It is considered that the remaining of nitrogen atoms (about 14%) may be converted into the nitrogen gas, thusly, the yield of total nitrogen was not 100%. The same type of result has been already reported in the photocatalytic degradation of other organic nitrogen compounds. Therefore, it was confirmed that the nitrogen atoms were almost mineralized under the solar irradiation. It is assumed that the ammonium ion could be generated when the intermediate products containing amino group (–NH2) are mineralized.

Fig. 8
figure 8

a Formation of NH4+ ions and (b) removal of TOC during the photocatalytic degradation of metalaxyl in presence of TiO2 under solar irradiation

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Generally, total organic carbon (TOC) indicates the amount of organic carbon dissolved in the aqueous solution. The decrease in TOC corresponds to the mineralization of organic carbon compounds in the aqueous solution. Therefore, the change of TOC in aqueous solution was examined during the photocatalytic degradation of metalaxyl. The results are shown in Fig. 8b. The theoretical TOC value of a 50 ppm metalaxyl solution is 32.25 ppm. From Fig. 8b, TOC decreased with the light irradiation time, and reached about 10 ppm after 1.5 h. About 70% of TOC could be removed within 1.5 h of light irradiation time in the presence of TiO2. The results indicate that the dissolved organic carbon was mineralized into CO2 by photocatalytic decomposition. Because after 5 h of solar irradiation, about 28% of TOC remained, the relatively stable intermediates may be formed during the photocatalytic treatment.

3.9 Identification of the intermediate products of metalaxyl photocatalytic degradation under solar degradation

It was found that metalaxyl can be degraded and mineralized by the photocatalytic action of TiO2. On the other hand, the various intermediates may be generated before the mineralization. Therefore, the intermediates from of metalaxyl was identified using a gas chromatography-mass spectrometer (GC–MS). The experimental conditions are given in supplementary information (Fig. S2). In the photocatalytic degradation, the reaction intermediates were extracted by solid phase extraction and analyzed by GC–MS. Five intermediate products has been identified by molecular ion and mass fragment peak and by comparing with GC/MS NIST library data. Based on the result obtained from GC–MS, the intermediate products are listed in Table 3. The intermediate products had more than 85% similarities for the NIST library data.

Table 3 Intermediates obtained during the photodegradation of metalaxyl
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3.10 A study on the probable mechanism of photocatalytic degradation of metalaxyl with TiO2

The photocatalytic degradation mechanism of metalaxyl is quite complex process. According to the present study and literature reports, the photocatalytic degradation mechanism of metalaxyl with TiO2 would be represented in Scheme 1. Most of the organic pollutants can degrade under sun light irradiation in presence of suitable metal oxide-based semiconductors. The nanosized TiO2 with the bandgap of 3.2 eV can efficiently absorb the photons with energy-greater than the band-gap one, and can be excited to form electrons in the conduction band (CB) and holes in the valance band (VB) under the solar irradiation [50] (Eq. 4). The oxidizing species (such as H2O and OH−) can be reacted with the valence band holes, h +VB , to form hydroxyl radical (·OH) (Eq. 5), and on the other hand, the conduction band electron can be trapped by reducing species (O2 and H2O2) from the reaction solution to generate mainly superoxide radicals (·O2−) or hydroperoxyl radicals (·OOH) on TiO2 surface [51, 52] (Eqs. 5–8). These generated radicals can degrade metalaxyl into CO2 and other products in aqueous solution (Eq. 10).

$$TiO_{2} + h\nu \to e_{CB}^{ - } + h_{VB}^{ + }$$
(4)
$$h_{VB}^{ + } + OH^{-} \left( {or H_{2} O} \right) \to \cdot OH \left( { + H^{ + } } \right)$$
(5)
$$e_{CB}^{ - } + O_{2} \to \cdot O_{2}^{-}$$
(6)
$$\cdot O_{2}^{-} + H^{ + } \to \cdot {\text{OOH}}$$
(7)
$$2 \cdot {\text{OOH}} \to H_{2} O_{2} + O_{2}$$
(8)
$$H_{2} O_{2} + e_{CB}^{ - } \to \cdot OH + OH^{ - }$$
(9)
$$\cdot OH \left( {or \cdot {\text{OOH}}} \right) + {\text{Metalaxyl}} \to \to Intermediate\,Products \to NH_{4}^{ + } + CO_{2} + H_{2} O$$
(10)

According to the MOPAC simulation (Table 2), 1C, 2C, 3C, 4C, 5C and 6C have comparatively higher frontier electron density. These sites would be attacked by neutral ·OH radicals. Especially, because of the steric hindrance, 1C, 2C and 6C could easily be attacked by ·OH radicals. Three probable degradation routes have been speculated roughly from the results of frontier electron calculations and the intermediate analysis. As a first route, hydroxyl radicals could attack the 1C, 2C or 6C of benzene ring, successively. Eventually, metalaxyl could be hydroxylated and can be converted into the intermediate 5 or its isomer. Thereafter, the benzene ring will be broken because of continuous attack of radical species. As the second route, C in alkyl group (R1) undergoes a radical attack and R1 would be hydroxylated. Then, two intermediates, namely intermediate 4 and 6, are formed. The intermediate 4 will be transformed into intermediate 3 by the attack of ·OH. Then, alkyl group (R2) of intermediate 3 is subjected to a radical attack, and intermediate 3 is converted into intermediate 1. Finally, the intermediate is mineralized into CO2 and ammonium ion. As the third route, alkyl R2 group would be attacked by ·OH, and the intermediate 2 is generated during the photodegradation. Consequently, in every route the aromatic ring will be broken, and therefore, ammonium ion and CO2 will be observed as final mineralization product.

Scheme 1
scheme 1

Probable pathways for the photocatalytic degradation of metalaxyl in presence of TiO2 under solar irradiation

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The photodegradation of metalaxyl involves mainly N-dealkylation and demethoxylation in addition to process such as rearrangement of the N-acyl group to another position of the benzene and the elimination of the methoxycarbonyl group and alanine methyl ester [53]. It is difficult to identify the aliphatic compounds formed from the benzene ring opening reaction by GC–MS analysis.

4 Conclusion

The present study revealed that metalaxyl was photocatalytically degraded under sunlight. The photocatalytic action of TiO2 can be considered as an effective and simple method for the remediation of metalaxyl contamination in water. The optimum conditions such as photocatalyst dosage, initial pH, light intensity, reaction temperature and light irradiation time was evaluated from the solar photodegradation of metalaxyl. Typically, the most optimal degradation conditions were photocatalyst dosage: 20 mg, temperature: 20 °C and pH: 6. The pseudo-first-order reaction kinetics were determined according to Langmuir–Hinshelwood model, with a degradation rate constant of 0.105 min−1. The degradation of metalaxyl using TiO2 is mainly an oxidative reaction by radical species such as hydroxyl radicals. Although metalaxyl is photo-degradable under sunlight, the addition of TiO2 could immensely accelerate the degradation efficiency of metalaxyl. At the end of reaction, metalaxyl is almost mineralized, and ammonium ion and CO2 were found as final product. Hence, in the future, this research can be expected as a practical degradation and detoxification technology of metalaxyl.